The overall objective of our grant is to define the effect of genetic and environmental factors on the survival time of x-irradiated mice undergoing graft-vs-host (GVH) after reconstitution with H-2 disparite bone marrow cells. One aim was to establish the number of independently segregating genes which controlled products previously assigned to the presumed single gene, Mls. Clarification of the ambiguities of Mls is an important point in our studies of the suspected ability of Mls products to modulate GVH induced by H-2 disparite cells.